(1) Field of the Invention
The present invention relates to an integrated semiconductor laser diode module and a manufacturing method thereof. More specifically, it relates to an integrated semiconductor laser diode module having a long lifetime and capable of low-threshold lasing and high output operation, and to a manufacturing method of such an integrated laser diode module.
(2) Description of the Related Art
In recent years, high capacity optical disc systems using red lasers as a light source for discs such as DVD-ROMs, DVD-RAMs, DVD-RWs, and DVD-Rs have been expanding the market. Further, various attempts in developing next generation high capacity optical disc systems using blue-violet lasers such as a Blu-ray disc system and an HD-DVD disc system have been made aggressively, with an increase of expectations for such disc systems. In developing such a next generation disc system, ensuring a compatibility with disc systems of the previous generation is an important key. Specifically, the DVD system had to be compatible with the CD system, and the Blu-ray and HD-DVD systems are required to be compatible with the DVD and CD systems. In order to achieve such compatibility, it is necessary that a next generation system is equipped with lasers having different wavelengths: 400 nm band, 650 nm band, and 780 nm band.
One solution to ensure the compatibility with the previous-generation system can be achieved by assembling different optical systems for different wavelengths in one disc system. However, this solution increases the production cost. Anew solution that has been gaining attention is to use integrated semiconductor laser diode modules capable of emitting laser beams having two different wavelengths.
As one example, Japanese Laid-Open Patent Application No. 2002-118331 discloses such an integrated semiconductor laser diode module structured by a 400 nm band laser diode made of gallium nitride (AlGaInN) material and a 650 nm band laser diode made of aluminum gallium indium phosphide (AlGaInP) material that are joined together. The following explains the integrated semiconductor laser diode module disclosed in the JP 2002-118331.
FIG. 1 is a perspective view of the conventional integrated semiconductor laser diode module in which a 400 nm band laser diode made of gallium nitride (AlGaInN) material is joined to a 650 nm band laser diode made of aluminum gallium indium phosphide (AlGaInP) material.
This module is structured by a 400 nm band laser diode LD 3310 and a 650 nm band laser diode LD 3330 that are joined together by a joining member 3341. The joining member 3341 is made of a conductive material such as metal. The LD 3310 includes a sapphire substrate 3301 and a gallium nitride semiconductor layers laminated on the sapphire substrate, and the LD 3330 includes a GaAs substrate 3321 and an aluminum gallium indium phosphide semiconductor layers laminated on the GaAs substrate 3321. The LD 3310 and LD 3330 are ridge waveguide type, for example.
FIG. 2 is a sectional view of the LD 3310, taken at line of Y-Z shown in FIG. 1. The LD 3310 is such that an n-type contact layer 3401, an n-type clad layer 3402, an emission layer 3403, a p-type clad layer 3404, and a p-type contact layer 3405 are laminated on one main surface of the sapphire substrate 3301. The layers 3401-3405 are made of gallium nitride semiconductor. The p-type contact layer 3405 and the p-type clad layer 3404 are partially removed down to the middle of the p-type clad layer 3404, so as to form a ridge. A dielectric layer 3406 as a current blocking layer is formed on a side surface of the ridge and on the p-type clad layer 3404. Further, a p-side ohmic electrode 3311 and a p-side pad electrode 3312 are formed on the ridge. In addition, the LD 3310 includes an n-side ohmic electrode 3351 and an n-side pad electrode 3352 formed on an n-type electrode forming region, where the layers 3401-3405 are partially removed down to the middle of the n-type contact layer 3401.
FIG. 3 is a sectional view of the LD 3330, taken at line of Y-Z shown in FIG. 1. The LD 3330 is such that an n-type clad layer 3501, an emission layer 3502, a p-type clad layer 3503, and a p-type contact layer 3504 are laminated on one main surface of the GaAs substrate 3321. The layers 3501-3504 are made of aluminum gallium indium phosphide semiconductor. The p-type contact layer 3504 and the p-type clad layer 3503 are partially removed down to the middle of the p-type clad layer 3503, so as to form a ridge. A dielectric layer 3505 as a current blocking layer is formed on a side surface of the ridge and on the p-type clad layer 3503. Further, a p-side ohmic electrode 3331 and a p-side pad electrode 3332 are formed on the ridge. In addition, an n-side ohmic electrode 3361 and an n-side pad electrode 3362 are formed on the other main surface of the GaAs substrate 3321.
The LD 3310 and LD 3330 are joined together with the joining member 3341, thereby structuring the integrated semiconductor laser diode module as illustrated in FIG. 1. The p-side pad electrodes 3312 and 3332 have the same potential because the p-side pad electrodes 3312 and 3332 are connected to each other by the joining member 3341 that is conductive, and the p-side pad electrode 3312 of the LD 3310 is used as a common electrode. By supplying the current between the p-side pad electrode 3312 and n-side pad electrode 3352, the LD 3310 is operated and emits 400 nm band laser beams. Further, by supplying the current between the p-side pad electrode 3312 and n-side pad electrode 3362, the LD 3330 is operated and emits 650 nm band laser beams.
It is extremely difficult to join the LD 3310 and LD 3330 with matching both directions of the ridges and the crystallography directions between the LD 3310 and LD 3330. This also makes it extremely difficult to make the facets flat for both laser diodes with the conventional manufacturing method, by which all facets of the resonators are formed at the same time. As shown in FIG. 1, if the facets are formed based on cleavage planes of the semiconductor that structures the LD 3330, numerous stripes due to uneven surface appear on the facets of the resonator of the LD 3310. The following explains about this problem in detail with reference to FIGS. 4-9, by showing an example of steps for manufacturing the conventional integrated semiconductor laser diode module.
As shown in FIG. 4, the layers 3401-3405 made of the gallium nitride semiconductor are laminated on the sapphire substrate 3301. Then, as shown in FIG. 5, a ridge 3701 in a stripe shape is formed using a common photolithographic technique and an etching technique, and then the dielectric layer 3406, p-side ohmic electrode 3311, and p-side pad electrode 3312 are formed respectively at desired regions.
Further, as shown in FIG. 6, the layers 3501-3504 made of the aluminum gallium indium phosphide semiconductor are laminated on the GaAs substrate 3321. Then, as shown in FIG. 7, a ridge 3901 in a stripe shape is formed using the common photolithographic technique and etching technique, and then the dielectric layer 3505, p-side ohmic electrode 3331, and p-side pad electrode 3332 are formed respectively at desired regions.
Next, as shown in FIG. 8, the wafers are joined by the joining member 3341 so that the p-side pad electrodes 3312 and 3332 face each other. At this time, it is difficult to join the wafers so that the stripe shaped ridges are accurately in parallel. FIG. 9 is a schematic view that illustrates this problem. As shown in the drawing, the wafers are not in parallel in the direction of the ridges (3701 and 3901), and an error Δθ in angle occurs. Specifically, the crystallographic directions of the wafers are not aligned. When forming a resonator by cleaving after joining two wafers in such a way, it is not possible to obtain flat cleavage planes for both laser diodes. As a result, the facets of the resonator of the laser diode (LD 3310) become uneven.
As described above, it is not possible, with the conventional integrated semiconductor laser diode module, to form all of the facets of the laser diodes integrated in one module evenly, which is ideal. The uneven facets make light guided inside a laser diode scatter, and also makes it difficult to cause laser oscillation. This causes an increase in a threshold current. In addition, the unevenness on the facets could increase nonradiation centers on the facets, which results in higher light absorption at the facets. Consequently, the facets could deteriorate and it becomes difficult to perform high output operation. Further, these factors give an adverse effect to the lifetime of the laser diodes.
Moreover, in the case of the integrated semiconductor laser diode module shown in FIG. 1, the AlGaInN laser diode and an AlGaInP laser diode are joined facing each other. Because both coefficients of thermal expansion (CTE) and crystal structures are greatly differ between an AlGaInN material and an AlGaInP material (the AlGaInN material has a wurtzite structure, while the AlGaInP material has a zincblende structure), heat generated when driving one laser could distort the other laser. As a result, this could shorten the lifetime of the laser diodes.